1. Field of the Invention
The present intention relates to acicular or needle-like oxide crystals (generally referred to as xe2x80x9coxide whiskersxe2x80x9d) and methods of producing the crystals. The present invention also concerns apparatus using the acicular oxide crystals (e.g., photoelectric conversion apparatus). Since the acicular oxide crystals have sharpness of their distal end, monocrystallinity, and large surface areas, they are expected to be applied in a variety of fields.
2. Related Background Art
Acicular crystals generally refer to monocrystalline bodies of an acicular shape having the diameter of not more than 100 xcexcm and the ratio of length to diameter (aspect ratio) of not less than 10.
Japanese Patent Application Laid-Open No. 5-139900 (U.S. Pat. No. 5,279,809) describes a method of producing an acicular crystal of zinc oxide.
Specifically, a source material is prepared by adding powder of a tin-zinc alloy to zinc powder having an oxide film on the surface. The source material is put into a crucible and heated to be oxidized. After that, the source material is cooled down to room temperature. Acicular crystals are produced in the crucible in this way.
Although the acicular crystals were produced in the crucible by the technology described in the above application, the acicular crystals were unable to grow on a surface of a substrate located outside the crucible.
The acicular crystal powder was synthesized by the above method of heating metal to oxidize and aggregate in vapor phase, but it was infeasible to grow an acicular crystal film on a substrate by the method. It was possible to produce an oriented acicular crystal film by the atmospheric CVD technique using an organic source material, but this technique had problems that the substrate was limited to a sapphire substrate, the acicular crystal film obtained was limited to only that of ZnO having the diameter of about several xcexcm, and cost was high because of use of the organic source material.
An object of the present invention is to provide a method of growing an acicular oxide crystal on a substrate, and photoelectric conversion apparatus using the acicular crystal produced by the method.
A method of producing acicular oxide crystals (i.e., oxide whiskers) according to the present invention is a method of producing acicular oxide crystals, comprising heating a source material comprising a metal or an inorganic compound at a first temperature to be vaporized and depositing a crystal constitutive material vaporized from the source material on a substrate heated at a second temperature lower than the first temperature.
The present invention is also characterized in that a source material comprising Zn as a main component is evaporated at the first temperature and transported onto the substrate located in an atmosphere containing oxygen, and acicular crystals of ZnO extending in the c-axis direction and having an average diameter of not more than 300 nm and aspect ratios of not less than 50 are formed on the substrate.
Particularly, it is preferable that a partial pressure of oxygen or a partial pressure of water vapor in the vicinity of the substrate be 100 to 10000 Pa and a temperature of the atmosphere in the vicinity of the substrate be 400 to 900xc2x0 C.
It is also preferable that a pressure in the vicinity of the substrate be not less than 1000 Pa nor more than 100000 Pa.
A partial pressure of oxygen in the vicinity of the evaporating portion of the source material can be made lower than the partial pressure of oxygen in the vicinity of the substrate. The source material can be one having an average composition represented by ZnOx, where 0xe2x89xa6Xxe2x89xa60.5.
Here, the inorganic compound can be effectually selected from oxides, but it can also be selected from nitrides, chlorides, and so on (e.g., TiCl4 and FeCl2).
The acicular crystal in the present invention encompasses all of circular cylinders and circular cones, and variations thereof, e.g., truncated circular cones, circular cylinders with a sharp-pointed tip or with a truncated tip, and so on. Further, it encompasses triangular pyramids, rectangular pyramids, hexagonal pyramids, other polyhedral pyramids, those with a truncated tip; triangular prisms, rectangular prisms, hexagonal prisms, and other polyhedral prisms; or triangular prisms, rectangular prisms, hexagonal prisms, and other polyhedral prisms with a sharp-pointed tip or with a truncated tip; and polygonal structures thereof.
An electroconductive (hereinafter, simply referred to as xe2x80x9cconductivexe2x80x9d) layer is preferably present on the surface of the substrate. The conductive film is sometimes effective when it is a transparent electrode. Preferred acicular oxide crystals are acicular crystals 70% or more of which stand with an axis thereof being not less than 60xc2x0 relative to the substrate.
The acicular oxide crystals are effectively applied, particularly, to the photoelectric conversion apparatus.
A dye-sensitized photoelectric conversion device will be described below.
FIG. 10 is a cross-sectional view showing the schematic structure of a Graetzel-type photochemical cell using a dye-sensitized semiconductor electrode. In FIG. 10 numeral 44 designates a glass substrate, 45 a transparent electrode formed on a surface of the substrate, and 41 an anatase-type porous titanium oxide layer consisting of a porous joint body in which fine particles of titanium oxide are joined to each other. Numeral 42 designates a dye joined to surfaces of the titanium oxide particles, which acts as a light absorbing layer.
Numeral 43 denotes an electrolyte solution functioning as an electron donor, e.g., an electrolyte containing iodine (I). Light is injected into the cell from the left in the figure. The action of this solar cell is as follows. Namely, the incident light excites electrons in the dye and the electrons thus excited are efficiently injected into the titanium oxide semiconductor layer. The dye in the oxidized state after the transmission of electrons quickly receives electrons from the electrolyte to be reduced into the original state. The electrons entering the interior of titanium oxide migrate between fine particles through the mechanism of hopping conduction or the like to reach the anode. Iodine ions in the electrolyte solution, oxidized because of the supply of electrons to the dye, are reduced at the cathode to return into the original state.
The dye can be selected, for example, from organic dyes and natural dyes such as perylene, rose bengal, Santaline, Cyanin, and so on, and metal complexes such as zinc porphyrin, ruthenium bipyridyl {Ru(dcbpy)2(SCN)2, (N3:dcbpy=2,2-bipyridine-4,4xe2x80x2-dicarboxylic acid)}, and so on, and it is important that oxidized and reduced forms thereof be stable. It is also necessary that potentials of excited electrons in the light absorbing layer, i.e., excitation levels of the photo-excited dye be higher than the level of the conduction band of the n-type layer.